Two live attenuated PIV3 candidate vaccines are in Phase 1-2 trials in human infants and children, namely the cold-passaged 45 mutant (cp45) of the JS wildtype human PIV3 and a bovine PIV3 that is antigenically related to human PIV3. The two vaccine candidates are safe, infectious, immunogenic, and genetically stable in seronegative infants and young children. The cp45 virus replicates to very high titer in Vero cell culture grown on microcarriers. The Master Seed stock of the cp45 virus maintains its temperature sensitive (ts), cold-adapted (ca), and attenuation (hamsters) phenotypes despite its high level of replication in the Vero microcarrier cultures. Thus, the large scale production of the cp45 virus at this stage appears feasible. A complete cDNA copy of the JS strain of human PIV3 has been constructed with the intent to rescue infectious virus from RNA transcribed from this synthetic cDNA copy of the RNA virus genome. This cDNA has the almost exact coding sequence of the wild type virus except for engineered mutations introduced by site-directed mutagenesis. These mutations were introduced to permit the virus rescued from the cDNA to escape neutralization by monoclonal antibodies that are used to neutralize the infectivity of the helper virus. In addition, mutations that mark the DNA have been introduced to permit unequivocal identification of rescued virus. The JS PIV3 cDNA has been modified for intracellular transcription of a full length vRNA sense genome for rescue into an infectious virus. Two systems of rescue of virus have been developed; a helper virus-based rescue system and a plasmid-based rescue system. Attempts to recover infectious virus from this full length cDNA using the two helper systems are in progress.